Fast-detaching electrically insulated implant

ABSTRACT

This is an implant for placement in the human body and an assembly for so placing that implant. Most desirably, it is an implant for use in the vasculature of the human body and is used to occlude some space in that vasculature as a portion of a treatment regimen. The implant itself is preferably a component of a deployment device using an electrolytically severable joint. The implant component is at least partially covered with a highly resistive or insulative covering. The highly resistive or insulative layer or covering appears to enhance the susceptibility of the electrolytic joint to quick erosion and thus detachment of the implant. Although the implant itself is preferably a vaso-occlusive device, it may instead be a stent, a vena cava filter, or other implant which may be installed in this manner. The implant may be independently coated with insulative or resistive material or may be formed using a material with such as tantalum, which forms such an insulator or resistor in situ.

RELATED PATENT APPLICATION DATA

This application is a continuation reissue of U.S. reissue applicationSer. No. 10/900,901, filed on Jul. 27, 2004 (now abandoned), which is areissue application of U.S. application Ser. No. 09/668,033, filed onSep. 21, 2000, now U.S. Pat. No. 6,425,914, which is a continuation ofU.S. patent application Ser. No. 08/920,526, filed on Aug. 29, 1997, nowU.S. Pat. No. 6,156,061 the entirety of which is hereby incorporated byreference.

FIELD OF THE INVENTION

This invention is an implant for placement in the human body and anassembly for so placing that implant. Most desirably, it is an implantfor use in the vasculature of the human body and is used to occlude somespace in that vasculature as a portion of a treatment regimen. Theimplant itself is preferably a component of a deployment device using anelectrolytically severable joint. The implant component is at leastpartially covered with a highly resistive or insulative covering. Thehighly resistive or insulative layer or covering appears to enhance thesusceptibility of the electrolytic joint to quick erosion and thusdetachment of the implant. Although the implant itself is preferably avaso-occlusive device, it may instead be a stent, a vena cava filter, orother implant which may be installed in this manner. The implant may beindependently coated with insulative or resistive material or may beformed using a material with such as tantalum, which forms such aninsulator or resistor in situ.

BACKGROUND OF THE INVENTION

Implants may be placed in the human body for a wide variety of reasons.For instance, stents are placed in a number of different lumens in thebody. They may be placed in arteries to cover vascular lesions or toprovide patency to the vessel. Stents are also placed in biliary ductsto prevent them from kinking or collapsing. Grafts may be used withstents to promote growth of endotbelial tissue within those vessels.

Vena cava filters are implanted in the body, typically in the vena cava,to catch thrombus which are sloughed off from other sites within thebody and which may be in the blood passing through the chosen site.

Vaso-occlusive devices or implants are used for a wide variety ofreasons. They are often used for treatment of intra-vascular aneurysms.This is to say that the treatment involves the placement of avaso-occlusive device in an aneurysm to cause the formation of a clotand eventually of a collagenous mass containing the vaso-occlusivedevice. These occlusions seal and fill the aneurysm thereby preventingthe weakened wall of the aneurysm from being exposed to the pulsingblood pressure of the open vascular lumen.

Treatment of aneurysms in this fashion is significant improvement overthe surgical method typically involved. The surgical or extravascularapproach is a common treatment of intra-cranial berry aneurysm; it isstraightforward but fairly traumatic. The method involves removing ofportion of the cranium and locating the aneurysm. The neck of theaneurysm is closed typically by applying a specially sized clip to theneck of the aneurysm. The surgeon may choose to perform a sutureligation of the neck or wrap the entire aneurysm. Each of theseprocedures is performed by an very intrusive invasion into the body andis performed from the outside of the aneurysm or target site. Generalanesthesia, craniotomy, brain retraction, and a placement of clip aroundthe neck of the aneurysm all are traumatic. The surgical procedure isoften delayed while waiting for the patient to stabilize medically. Forthis reason, many patients die from the underlying disease prior to theinitiation of the surgical procedure.

Another procedure—the extra-intravascular approach—involves surgicallyexposing or stereotaxically reaching an aneurysm with a probe. The wallof the aneurysm is perforated from the outside and various techniquesare used to occlude the interior of the aneurysm to prevent itsrebleeding. The techniques used to occlude the aneurysm includeelectro-thrombosis, adhesive embolization, hoghair embolization, andferromagnetic thrombosis. These procedures are discussed in U.S. Pat.No. 5,122,136 to Guglielmi et al., the entirety of which is incorporatedby reference.

Guglielmi et al. further describes an endovascular procedure which is atonce the most elegant and least invasive. The procedure described inthat patent includes a step in which the interior of the aneurysm isentered by the use of guidewire such as those in Engelson, U.S. Pat. No.4,884,579 and a catheter as in Engelson, U.S. Pat. No. 4,739,768. Thesepatents described devices utilizing guidewires and catheters which allowaccess to aneurysms from remote parts of the body. Typically, thesecatheters enter the vasculature through an artery in the groin. TheGuglielmi et al system uses catheters and guidewires which have a veryflexible distal regions and supporting midsections which allow thecombinations to be steerable to the region of the aneurysm. That is tosay that the guidewire is first steered for a portion of the route tothe aneurysm and the catheter is slid up over that guidewire until itreaches a point near the distal end of the guidewire. By steps, thecatheter and guidewire are then placed at the mouth of the aneurysm. Thecatheter is introduced into the aneurysm and vaso-occlusive orembolism-forming devices may be delivered through the lumen.

Various vaso-occlusive devices are introduced through the notedmicrocatheters to close the aneurysm site. In some instances, a smallballoon may be introduced into the aneurysm where it is inflated,detached, and left to occlude the aneurysm. Balloons are becoming lessin favor because of the difficulty in introducing the balloon into theaneurysm sac, the possibility of aneurysm rupture due to over-inflationof the balloon within the aneurysm, and the inherent risk associatedwith the traction produced when detaching the balloon.

Another desirable embolism-forming device which may be introduced intoaneurysm using end of vascular placement procedure is found in U.S. Pat.No. 4,994,069 to Ritchart et al. In that patent are described variousdevices—typically platinum/tungsten alloy coils having very smalldiameters—which may be introduced into the aneurysm through a cathetersuch as those described in the Engelson patents above. These coils areoften made of wire having a diameter of 2-6 mils. The coil diameter isoften 10-30 mils. These soft, flexible coils, may be of any lengthdesirable and appropriate for the site to be occluded. After thesevaso-occlusive coils are placed in, e.g., a berry aneurysm, they firstcause a formation of an embolic mass. This initial mass is shortlythereafter complemented with a collagenous material which significantlylessens the potential for aneurysm rupture.

There are variety of other vaso-occlusive devices, typically coils whichmay be delivered to the vascular site in a variety of ways, e.g., bymechanically detaching them from the delivery device. A significantnumber of these devices are described in patents owned by TargetTherapeutics, Inc. For instance:

U.S. Pat. No. 5,234,437, to Sepetka shows a method of unscrewing ahelically wound coil from a pusher having interlocking surfaces.

U.S. Pat. No. 5,250,071, to Palermo shows an embolic coil assembly usinginterlocking clasps both on the pusher and on the embolic coil.

U.S. Pat. No. 5,261,916, to Engelson shows a combinationpusher/vaso-occlusive coil assembly joined by an interlocking ball andkeyway type coupling.

U.S. Pat. No. 5,304,195, to Twyford et al., shows apusher/vaso-occlusive coil assembly having a fixed proximally extendingwire carrying a ball on its proximal end and a pusher having a similarend which two tips are interlocked and disengaged when expelled from thedistal tip of the catheter.

U.S. Pat. No. 5,312,415, to Palermo shows a method for dischargingnumerous coils from a single pusher by using a guidewire which has asection capable of interconnecting with the interior of a helicallywound coil.

U.S. Pat. No. 5,350,397, to Palermo et al. shows a pusher having athroat at its distal end and a pusher through its axis. The pusherthroat holds onto the end of an embolic coil and releases that coil uponpushing the axially placed pusher wire against member found on theproximal end of the vaso-occlusive coil.

Other mechanically detachable embolism forming devices are known in theart.

Each of the patents listed herein is specifically incorporated byreference.

Guglielmi et al. shows an embolism forming device and procedure forusing that device which, instead of a mechanical joint, uses anelectrolytically severable joint. Specifically, Guglielmi et al.desirably places a finely wound platinum coil into a vascular cavitysuch as an aneurysm. The coil is delivered endovascularly using acatheter such as those described above. After placement in the aneurysm,the coil is severed from its insertion core wire by the application of asmall electric current to that core wire. The deliverable coils are saidto be made of a platinum material. They may be 1-50 cm or longer as isnecessary. Proximal of the embolic coil, as noted above, is a core wirewhich is typically stainless steel. The core wire is used to push theplatinum embolic coil into vascular site to be occluded.

Other variations of the Guglielmi et al. technology are found in U.S.Pat. No. 5,354,295.

None of the references described above teach or suggests an implanthaving a highly resistive or insulative layer on at least a portion ofits exterior surface which is flexibly attached to an electrolyticallyseverable delivery joint.

SUMMARY OF THE INVENTION

This invention is an implant which is at least partially coated with aninsulative material. The implant may be a vaso-occlusive device, stent,vena cava filter, or any other implant which may be delivered via acatheter. Desirably, the device includes a core wire having a distaltip, which distal tip may be introduced into the selected site. The corewire is attached to the distal tip or implant in such a way that it maybe electrolytically detached by application of a current to the corewire.

The improvement involves the use of an insulative or highly resistivecovering on at least a portion of the implant. The resistive covering ispreferably one which is formed in situ from the material making up theimplant. This insulative or highly resistive layer appears to focus thecurrent flow through the sacrificial electrolytic joint and therebyimproves the rate at which detachment occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the overall layout of a delivery system showing the typicalmajor parts for delivering an implant using the improvement of thisinvention.

FIG. 2 shows a partial close up of a variation of the invention.

FIGS. 3A, 3B, and 3C show, respectively, cross sections depictingvarious manners of coating implants made according to this invention.

FIG. 4 shows a partial cross section of an electrolytic joint and avaso-occlusive braid all made according to this invention.

FIGS. 5 and 6 show helically wound vaso-occlusive coils having secondaryshapes which may be deployed using the procedures described herein.

FIG. 7 shows a stent implant using the improvements of this invention.

FIG. 8 shows a vena cava filter which may be deployed using theimprovements of this invention.

FIGS. 9A and 9B show, in schematic form, a procedure for deploying theinventive vaso-occlusive device of this invention.

DESCRIPTION OF THE INVENTION

As noted above, the Guglielmi et al. system for deploying an implantinto the human body uses a core wire, an electrolytic sacrificial joint,and the implant to be deployed. A power supply is needed to providepower for electrolysis of the joint. The core wire is typicallyinsulated on its outer surface from near the proximal end of the wire tothe electrolytic sacrificial joint. The implant typically forms aportion of the circuit through the body. This invention substantiallyremoves the implant itself from that circuit, thereby apparentlyfocusing the current flow at the electrolytic joint where it is needed.

FIG. 1 shows a system for introducing and deploying the implant madeaccording to the invention. The system (100) includes a catheter (102)which is made up of elongate tubular member, typically made from apolymeric material and often reinforced to provide strength or obviatekinking propensities with a coil or braid. Catheter (102) is shown witha pair of radio-opaque markers (106). The radio-opaque markers (106)allow visualization of the distal end (104) of the catheter so tocompare it with the coincidence of the implant (114). Catheter (102) isalso shown with a proximal fitting (108) for introduction of dyes ortreatment materials. Within the lumen of catheter (102) is a core wire(110) extending both proximally of catheter (102) and distally. On thedistal end of core wire (110) may be seen the electrolytic joint (112)and the implant (114). In this instance, implant (114) is a helicallywound vaso-occlusive coil. Generally, all of core wire (110) iselectrically insulated from a point near the proximal end of core wire(110) continuously to electrolytically severable joint (112).Electrolytically severable joint (112) is bare and is relatively moresusceptible to electrolysis in an ionic solution such as blood or mostother bodily fluids than is the implant (114).

The most proximal end of core wire (110) is also left bare so that powersupply (116) may be attached. The other pole of the power supply (116)is typically attached to a patch (118). The patch (118) is placed on theskin to complete the circuit from the power supply (116), through thecore wire (110), through electrolytic joint (112), through the ionicsolution in the body, and back to a patch (118) to the power supply(116). Other return routes may be used as the designer sees fit.

FIG. 2 shows a close-up of the more distal portion of the core wire(110) and the attached implant (120). Typically, core wire (110) will beconductive but covered with a insulative layer both proximal and distalof electrolytically severable joint (112). The interior of core wire(110) is typically electrically and physically attached to implant(120). In this variation of the invention, implant (120) is shown to bea helically wound coil (130) with an end (132) and having astretch-resisting member (134) through its center lumen. Theanti-stretch member (134) may be of any suitable material, e.g.,metallic wire or polymeric threads. Preferred are polymeric threads ofpolypropylene or polyethyleneterephthalate, although almost anypolymeric material which is formidable into a fiber is quite suitable.The stretch resisting member is simply designed to prevent the coil(130) from stretching in the event that core wire (110) must bewithdrawn or repositioned to change the position of the implant (120).

The coil making up this variation of the invention is generally of adiameter in the range of 0.00025 inches and 0.006 inches. Wire of suchdiameter is wound into a primary form having a diameter of between 0.003and 0.025 inches. For most neurovascular indications, the preferableprimary coil diameter is preferably between 0.008 and 0.018 inches.

The axial length of the primary coil will usually fall in the range of0.5 to 100 cm, more usually 2.0 to 40 cm. Depending upon usage, the coilmay well have 10-75 turns per centimeter, preferably 10-40 turns percentimeter. All of the dimensions here are provided only as guidelinesand are not critical to the invention. However, only dimensions suitablefor use in occluding sites within the human body are included in thescope of this invention.

Central to this invention is the provision of a highly resistive orinsulative layer or covering on at least a portion of implant (120).Without wishing to be bound by theory, it is believed that the coveringon implant (120) prevents or lessens current flow through the implant(120) itself and concentrates the current flow through the electrolyticjoint (112). Preferably, implant (120) has at least 95% of its surfacearea covered with the layer. The layer, which will be discussed in moredetail below, should not be of a type which interferes with theformation of the occlusion, when the implant is an occlusion device. Itsimilarly should not interfere with the other functions inherent withthis specific type of implant placed distally of the electrolyticallyseverable joint (112). That is to say that, for instance, the insulativelayer should not interfere with the function of a stent by, e.g., beingthrombogenic.

FIGS. 3A, 3B, and 3C show cross sections of the element making up theimplant (140) made according to this invention. FIG. 3A shows implant(140) having a core (142) and a resistive or insulative cover (144). Inthis variation, the core (142) may be of an oxide-forming material. A“oxide forming material” is one which, under the imposition of anelectrical current, will form an oxide skin (144), particularly in anionic medium such as saline solution, blood, or other bodily fluids. Onesuch material is the metal tantalum and certain of its alloys. In thisinstance, the device such as shown in FIG. 1 and FIG. 2 may beintroduced into the body without an oxide or insulative layer. Theinsulative layer will form in situ upon application of a current. Thisis the preferred embodiment of the invention. This insulative layer,once formed, prevents current flow from the interior core (142) into theionic medium surrounding the implant (140) once layer (144) has beenformed.

The implant may be made of other insulation-forming materials or oxideforming materials including metals such as zirconium, its alloys, andrelated materials which form or may be made to form exterior resistivelayers by, e.g., nitriding, or the like, preferably but not necessarilyin situ.

Although the core (142) may be completely made of a insulation formingmaterial as is shown in FIG. 3A, the core (146) maybe of anothermaterial, e.g., platinum or the like. As shown in FIG. 3B, a covering ofan oxide or insulative forming material (148) such as tantalum or azirconium may be plated or sputtered onto the core (146). As was thecase with the variation shown in FIG. 3A, the insulative forming layer(148) will form an insulation layer (150) preferably upon application ofa suitable electric current in situ but not necessarily so.

FIG. 3C shows still another variation of the inventive device in whichcore (152) is simply covered with an insulative material (154), whichinsulative material (154) is preferably inorganic in nature. Forinstance, to utilize an implant made of a conductive material such asstainless steel or gold and prevent or substantially lessen the flow ofelectrical current from the implant (140) into the surrounding ionicmedium, an inorganic covering may be applied to the exterior surface.The difference between the covering (154) found in FIG. 3C and theexterior coverings found in FIGS. 3A and 3B (144, 150) is that theformer coverings are capable of being formed in situ while the lattercovering in FIG. 3C is not.

Although the preferred variation of the invention is that found in FIG.2 wherein a helical coil constructed of tantalum is fixably attacheddistally of electrolytically severable joint (112), other forms ofimplants are suitable. For instance, FIG. 4 shows another variation ofthe inventive device (160) in which the implant is a vaso-occlusivebraid (162), either woven or unwoven. The electrolytically severablejoint (164) is shown to be located proximally of vaso-occlusive braid(162). In this variation, the core wire (166) is insulated with atubular member (168) and a bushing (170). A stabilizing coil (172) isalso depicted on the distal end of core wire (166). Bushing (170) andcovering (168) serve to electrically insulate core wire (166) from thesurrounding ionic fluid. These coverings along with those found onvaso-occlusive woven braid (162) are considered to focus theelectrolysis process on the electrolytically severable joint (164).

When the implant is a vaso-occlusive device, the shape of the device maybe any of a number of suitable overall shapes to promote occlusion ofthe selected interior body space. In particular, when the implant is ahelical coil, many shapes are known for treatment of particularabnormalities. FIGS. 5 and 6 show useful devices for treatment ofarterio-venous malformations (AVM) and aneurysms. Specifically, FIG. 5shows a vaso-occlusive coil which has a secondary conical shape. A“secondary” shape is meant to include any form in which a wire is firstformed into a first helical form and that first helical form is woundinto a second form, possibly helical. As was noted above, vaso-occlusivedevices are introduced through a catheter. Pushing the vaso-occlusivedevice through the catheter uses that first linear configuration whichapproximates the shape of the interior of the catheter. Secondaryshapes, such as shown in FIGS. 5 and 6, are formed when thevaso-occlusive device is allowed to exit the distal end of the catheter.The secondary shape of the vaso-occlusive device (180) shown in FIG. 5is, as noted just above, conical in form.

FIG. 6 shows a variation (182) of the inventive device in which twosections of the catheter have different secondary diameters.

Each of the catheters described herein may also have attached fibrousmaterials to increase thrombogenicity.

FIG. 7 shows a variation (190) of the inventive device in which theimplant is a stent (192). Core (194) is also is shown with anelectrolytically erodable joint (196).

The stent shown in FIG. 7 is a variation of a self-expanding stenttypically made of a super-elastic alloy material, typically anickel-titanium alloy (e.g., nitinol), that is well known in the art.The device is shown to have a zig-zag pattern of a metallic wire whichis maintained in the noted and secondary form by a filament (198) whichis woven through the bends of the stent to maintain the secondary shape.The primary shape is simply the shape shown but with a significantlylower diameter. The form of the stent is not important to the inventionbut is only illustrative of the form an implant may take.

FIG. 8 shows the expanded form of an implant (200) which may be used asa vena cava filter. Vena cava filters are well known in the art and areused to prevent the flow of blood clots distally in the vasculature.These blood clots would eventually be the site of occlusive strokes inthe brain if allowed to travel distally. In any event, implant (200)shows the vena cava filter member (202), the electrolytically severablejoint (204) and the push wire or core wire (206).

FIGS. 9A and 9B show placement of the inventive devices, specificallythe vaso-occlusive variations of the invention, within the human body.FIG. 9A shows the placement within a vessel (200) with a tip of catheter(202) placed near aneurysm neck (204). The aneurysm itself is nominated(206). Vaso-occlusive device (208) is fed into aneurysm (206) at leastuntil the sacrificial link (210) (hidden within catheter (202) at thisstep) is exposed beyond the distal tip of catheter (202). A positiveelectric current of approximately 0.01-2 milliamps at 0.1-6 volts isapplied to core wire (212). Thrombus is then formed within aneurysm(206). The negative pole (214) of power supply (216) is typically placedin electrical contact with the skin so to complete the circuit.

After the thrombus (218) has been formed and the aneurysm occluded,vaso-occlusive device (208) is detached from core wire (212) byelectrolytic disintegration of sacrificial link (210).

After sacrificial link (210) is at least mostly dissolved byelectrolytic action, typically in less than two minutes and most oftenin less than one minute, the core wire (212) and catheter (202) areremoved from vessel (200) leaving aneurysm (206) occluded as shown inFIG. 9B.

This procedure is practiced under fluoroscopic control either withgeneral or local anesthesia. A transfemoral catheter is typically usedto treat cerebral aneurysms and is usually introduced at the groin. Whenthe vaso-occlusive device (208) is insulated or covered with a highlyresistive material as is contemplated this invention, it is not affectedby electrolysis. When the core wire (212) and the pertinent portions ofthe supporting coils at the distal tip of the core wire (when utilized)are adequately coated with insulating coverings, only the exposedportion of the sacrificial link (210) is affected by the electrolysis.

Many alterations and modifications may be made by those having ordinaryskill in this art without departing from the spirit and scope of theinvention. The illustrative embodiments have been used only for thepurposes of clarity and should not be taken as limiting the invention asdefined by the following claims.

We claim the following:
 1. A method for introducing an implant member into a selected space within the human body comprising: placing an implant member via an attached core wire into the selected space, said implant member having a proximal end and a distal end and having an implant member core at least partially covered with an inorganic insulative covering comprising platinum, the implant member core being substantially covered with one of tantalum or zirconium, wherein the implant member further comprises an electrolytically severable joint attached to the implant member proximal end, the joint being relatively more susceptible to electrolysis in an ionic solution than is the implant member, and electrolytically detaching the implant member from the core wire.
 2. The method of claim 1 wherein the implant member further comprises an electrolytically severable joint attached to the implant member proximal end, the joint being relatively more susceptible to electrolysis in an ionic solution than is the implant member, and the step of detaching the implant from the core wire is accomplished electrolytically.
 3. The method of claim 1 wherein the implant member at least partially comprises an oxide-forming material.
 4. The method of claim 1 wherein the implant member comprises a core at least partially covered by a oxide-forming member.
 5. The method of claim 2 wherein the implant member comprises a core at least partially covered by an oxide-forming material 1, wherein at least 95% of the surface area of the implant member is covered with one of tantalum or zirconium.
 6. The method of claim 1 wherein the implant member comprises a stent.
 7. The method of claim 1 wherein the implant member comprises a vena cava filter.
 8. The method of claim 1 wherein the implant member comprises a helically wound coil.
 9. The method of claim 8 wherein the coil further comprises a stretch resistant member extending at least partially from a proximal end of the coil to a distal end of the coil.
 10. The method of claim 9 wherein the stretch resistant member comprises polypropylene.
 11. The method member of claim 1 wherein the implant member comprises a braid.
 12. A method for occluding a selected space within a human body comprising: placing the a tip of a catheter near the selected space, introducing into a lumen of said catheter a tantalum an occluding member having a proximal end and a distal end and including an inner core comprising platinum, the inner core being substantially covered with one of tantalum or zirconium and further comprising an electrolytically severable joint attached to the occluding member proximal end, the joint being relatively more susceptible to electrolysis in an ionic solution than is the occluding member, placing the occluding member into the space through the catheter tip, and electrolytically severing the joint.
 13. The method of claim 12 wherein the occluding member is at least partially covered with tantalum oxide.
 14. The method of claim 12 wherein the occluding member comprises a coil.
 15. The method of claim 14 wherein the coil further comprises a stretch resistant member extending at least partially from a proximal end of the coil to a distal end of a coil.
 16. The method of claim 12 wherein the occluding member comprises a braid.
 17. A method for occluding an aneurysm comprising: placing the a tip of a catheter near the aneurysm via a blood vessel, introducing into a lumen of the catheter a vaso-occlusive coil having a proximal end and a distal end and a highly resistive or insulative layer on at least a portion of the coil, the coil including an inner core comprising platinum, the inner core being substantially covered with one of tantalum and zirconium, the coil further comprising a sacrificial link attached to the coil proximal end, the link being relatively more susceptible to electrolysis in an ionic solution than is the coil, said link connected to a core wire, advancing the coil through the catheter lumen, out the catheter tip and into the aneurysm, applying a positive electric current to the core wire to dissolve the link,; and removing the core wire and catheter from the vessel.
 18. The method of claim 17 wherein the coil comprises tantalum.
 19. The method of claim 18 wherein the coil is at least partially covered with tantalum oxide.
 20. The method of claim 17 wherein the core wire comprises platinum.
 21. A method for introducing an implant member into a selected space within the human body comprising: placing an implant member via an attached core wire into the selected space, the core wire having a distal end comprising an electrolytically severable joint, said implant member comprising a proximal end and a distal end and a core formed from platinum, the core being substantially covered with one of tantalum and zirconium; applying an electrical current to the core wire, wherein said current transforms the one of tantalum and zirconium into an insulative oxide in situ on an exterior surface of the implant member; and detaching the implant member from the core wire at the electrolytically severable joint.
 22. The method of claim 21 wherein the implant member comprises a stent.
 23. The method of claim 21 wherein the implant member comprises a vena cava filter.
 24. The method of claim 21 wherein the implant member comprises a helically wound coil.
 25. The method of claim 24 wherein the coil further comprises a stretch resistant member extending at least partially from a proximal end of the coil to a distal end of the coil.
 26. The method of claim 25 wherein the stretch resistant member comprises polypropylene.
 27. The method of claim 21 wherein the implant member comprises a braid. 